For example, in a projector as an optical device used for image display like a liquid crystal projector or a DLP (Trademark) projector, a high intensity discharge lamp (HID lamp) is used. In such a projector, by a dichroic prism etc., light is separated into the three primary colors of red, green, and blue, i.e., a space modulation element provided for each color generates an image of each of the three primary colors, and optical paths thereof are combined by a dichroic prism etc., so as to display a color image. In another known type of projector, light emitted from a light source is passed through a rotating filter (dynamic color filter) comprising a transmission color wheel having three primary color areas (R, G, and B), thereby sequentially generating light rays of the three primary colors. In synchronization with the generated light rays, the spatial modulation device is controlled so as to sequentially generate an image of each of the three primary colors in a time dividing manner, thereby displaying a color image.
Referring to FIG. 14 in which (a) shows change of lamp voltage (VL) and (b) shows change of lamp current (IL), a lighting operation of a discharge lamp lighting apparatus for a discharge lamp which is described above is explained briefly. First, in a state where voltage called a no-load open circuit voltage is impressed to the lamp at start-up (at time (to) or after in FIG. 14), high voltage etc. is impressed to electrodes, to generate dielectric breakdown in an electrical discharge space, and the state of discharge changes from glow discharge (at time (tg) or after in FIG. 14) to arc discharge (at time (ta) or after in FIG. 14), and finally, the discharge lamp lighting apparatus operates so that stable regular lighting may be realized.
The lamp discharge voltage which was low, i.e. about 10V, immediately after shifting from the glow discharge to the arc discharge, that is, immediately after the time (ta) of FIG. 14, goes up gradually in connection with a temperature rise, and is stabilized at constant voltage in a lighting steady state. Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply for the lamp discharge voltage, in order to realize predetermined input electric power applied to the lamp thereby outputting required lamp current. Moreover, the lamp voltage, i.e., an output voltage of the converter, is detected. Based on information of the voltage detection, the target lamp current is determined according to a value of the quotient which is obtained by dividing, for example, target electric power by the detected voltage. Since, during a period when the lamp voltage immediately after shifting from the glow discharge to the arc discharge, is low, the target lamp current which is computed by dividing the target electric power by the detected voltage becomes very large (value), it cannot be realized. Instead of it, the maximum value ILmax of the lamp current is adopted as the target lamp current, so that when the lamp voltage goes up, and the calculation value obtained by dividing the target electric power by the detection voltage, becomes less than the maximum value Ilmax. Thus, the target lamp current is set as the calculation value.
As types of a driving method of such a discharge lamp, there are a direct-current drive method in which a lamp is turned on by a converter, and an alternating current driving method in which periodic polarity reversals (inversions) occur by providing an inverter further in the downstream side of the converter. In the case of the direct-current drive method, since the light flux from a lamp is like direct current, that is, it does not change with passage of time, basically, there is a big advantage that it can be similarly applied to both types of the above described projectors. On the other hand, in the case of the alternating current driving method, by using the flexibility of polarity-reversal frequency, that the direct-current drive method does not have, there is an advantage that growth or consumption of electrodes of the discharge lamp may be controlled. On the other hand, there are disadvantages resulting from existence of polarity reversals i.e., overshoot, instantaneous light-out at the time of polarity reversals etc. occurs thereby causing a bad influence on a display image etc.
On the other hand, when a dynamic color filter is used, in order to obtain high color-reproduction performance of a display image, it is important to adjust spectrum distribution of a light source lamp to the form of conversion to the color sequential light flux. In the case of the color wheel, it is possible to improve a color-reproduction performance or to carry out a desired color-reproduction performance by setting up angle distribution of R, G, and B areas of the color wheel (depending on circumstances, W (white) is added to the R, G, B), that is, by setting up the rate of a period per rotation during which light transmits through each color area, according to the spectrum of the lamp.
For example, when a R component runs short, it is effective to make the rate of a period during which light transmits through the R area longer than the rates of the other color periods. However, in the DLP system type projector, in order to obtain a desired color-reproduction performance by such a method, since the brightness for each color of each pixel of a display image is controlled by the duty cycle ratio, in process of each pixel of a space modulation element, there is a problem that it is difficult to carry out fine control of pixel tone, in the color component for which the rate of the period during which light currently transmits through the color area is reduced.
In order to solve such a problem, for example, in Japanese Laid Open Patent (Tokuhyo) No. H08-505031, it is proposed that, in an image projection apparatus, a light source drive control unit which changes an output power of a light source, synchronizing with the color of an optical beam outputted from a color change unit is provided. Similarly, in Japanese Laid Open Patent No. 2004-526992, a color-display apparatus in which electric power having two levels corresponding to colors is supplied, is proposed. Furthermore, in Japanese Laid Open Patent No. 2005-353343, without respect to the polarity-reversal timing of the lamp current which is driven in a manner of an alternating current, a projector which superimposes the pulse-current, synchronized with the color of a rotation filter, on lamp current is proposed.
In short, the common feature of such technology is to carry out modulation so that a pulse may be superimposed on lamp current corresponding to the filter color which appears in the projector. However, there is a problem which is not solved in these proposals.
Since lamp voltage of a high intensity discharge lamp is low immediately after start-up as described above, in order to form the light flux of a lamp fast and to make it shift to a steady state fast, it is necessary to pass the largest possible lamp current in the period immediately after the start-up so that heating of a lamp may be accelerated. However, the absolute value of the current which can be passed at this time has a maximum value ILmax, and if the current exceeding it is passed therethrough, even though it is pulse-current, consumption of lamp electrodes is caused. Therefore, during the period, under the condition where it does not exceed the maximum value ILmax of the lamp current, the average of lamp current increase so that the light flux of a lamp may be formed fast if a pulse is not superimposed rather than superimposing a pulse. This is because when a pulse is not superimposed, the lamp current can be passed continuously up to the full maximum value ILmax. On the other hand, in the case the pulse is superimposed, the lamp current is smaller than the maximum value ILmax on some level in periods other than the period when the pulse is superimposed.
Therefore, in the period immediately after start-up, the function for superimposing a pulse on lamp current is suppressed. That is, the function of a pulse superposition could not but be controlled to be released after waiting until a lamp is heated up so that lamp voltage goes up, and the lamp current becomes less than the maximum value ILmax on some level, so that it becomes in a state where it is certain that a peak current value will not exceed the maximum value ILmax even though a pulse is superimposed. However, in such a release of the suppression, since it is suddenly shifted from the state where the control is carried out to the state where the control is released, so that a viewer of the projection image of a projector perceives rapid change of the tone and the brightness of an image. Therefore, there is a big problem that it is very offensive to the eyes of the viewers. Moreover, in certain instances, there is a problem that it gives the viewer excessive uneasiness, by, for example, mixing up it with failure of machine.
In order to ease such problems, the control of a pulse superposition function is released as early as possible, so that the timing of the release is set at the time when the projection image of a projector is not much bright, that is, before a lamp is heated enough, whereby it is necessary to make it hard to see the rapid change of the image at the time of the release of the restraining. Therefore, since release of the control of the pulse superposition function is forced at time when the lamp voltage is not enough, there is almost no extra room to the maximum value ILmax of lamp current or depending on variation in a lamp, there is a problem in which the pulse is superposed on the lamp current so as to exceed the peak value. Thus, it is not ideal from a viewpoint of a lamp life.